1. Field of the Invention
The present invention relates to an image forming system using an electrophotographic process, and more particularly to a method of forming images on both sides of a recording paper and an image forming apparatus having a double-side image forming function.
2. Description of the Prior Art
An electrophotographic printer or a copying machine having a double-side image forming function has been proposed from the viewpoints of resource-saving and paper-saving in an office. In Japanese Patent Laid-open Publication No. SHO 59-222860, a double-side image forming system is disclosed in which two image forming units are provided in a single printer and respective images are formed on both sides of a recording paper by the two image forming units. However, since the apparatus requires two image forming units, it becomes a large size, as is a conventional copying machine, and the cost also becomes higher. Then, in recent years, there has been extensively adopted a switchback method in which an image is formed on the obverse of a recording paper, the recording paper is then reversed by a switchback, and an image is formed on the reverse of the paper (Japanese Patent Laid-open Publication No. SHO 60-114876, Japanese Patent Laid-open Publication No. SHO 63-54060).
The switchback method, however, has a complicated paper conveying path and requires a considerably wide space for reversing a recording paper. Moreover, since the size of this necessary space is proportional to the size of the recording paper, a printer capable of handling a large paper is inevitably increased in size. Also, since, after formation of an image on the obverse of the recording paper, the paper is reversed and an image is formed on the reverse of the paper, the paper conveying path becomes longer and it is difficult to form an image at high speeds. Further, conveying a recording paper complicatedly becomes a cause of paper jamming and various countermeasures therefor are also needed.
An important object of the present invention is accordingly to provide a double-side image forming method which is capable of forming images on both sides of a recording paper without reversing the obverse and reverse thereof.
Another important object of the present invention is to provide an image forming apparatus capable of forming an image at high speeds without using a switchback.
Still another important object of the present invention is to provide an image forming apparatus which is a small size and capable of forming an image on one side and both sides of a recording paper.